The following invention relates generally to fasteners. More particularly, this invention pertains to the field of fasteners having an exterior thread located peripherally about the shaft of the fastener, in which the thread spirals along a portion of the shaft. Most specifically, the thread, when taken in cross-section has a contour which increases in a dimension as it extends radially away from a root of the thread.
Threaded fasteners are extremely old devices which exhibit superior holding properties over non-threaded fasteners primarily due to a higher coefficient of friction. This coefficient of friction is manifested along the inter-face of the threads to its connecting; environment. Fasteners where the threaded portion exhibits a dovetail when viewed in cross-section define one specie of threaded fastener.
The following patents reflect the state of the art of which applicant is aware and is included herewith to discharge applicant""s avowed duty to disclose prior art. It is respectfully submitted, however, that none of these patents singly nor when combined in any conceivably permissible manner teach or render obvious the instant invention as set forth hereinafter.
While several of these patents teach the use of an element which appears to benefit from dovetail geometry, none provide interconnection between a threaded fastener and a correspondingly contoured bore in which the fastener meets with gradually increased resistance, providing an interference fit as the fastener becomes progressively engaged.
This invention is a new type of thread combination that has a special ability to self tighten while creating a radial clamping action. Normal straight threads do not have an ability to self tighten or an ability to clamp radially. Normal threads in fact create radial spreading force when tightened into a threaded bore or nut. This radial spreading force often overpowers the strength of the piece it is tightened into and causes the piece to crack. This is very common in cast iron and aluminum castings with tapped holes. The object of this invention is to provide a means wherein both of these limitations of normal threads are overcome. With this thread design applied to a tapered pipe lug and receiving hole in a casting, problems of over tightening the plug to obtain a seal which often results in a crack being formed in the side wall of the treaded hole will be eliminated.
The thread can also be applied to metal stitching pins used to repair cracked and damaged castings. One advantage over the prior art is that the old method which required a machined counterbore to be cut into the surface of the casting is eliminated. The present invention will make it much easier to repair thin wall castings that currently are very difficult to repair with hook thread style stitching pins.
The prior art also teaches crack repair utilizing either tapered threaded pins or straight threaded pins having a shoulder or abutment above the threads to halt the forward rotation of the pin thus causing it to tighten in the threaded bore. This new invention does not require either a shoulder or abutment.
The thread design in cross-section resembles that of an interlock wherein the thread cross-section of the crest or major outside surface of the thread tooth is wider than the cross-section of the root thread tooth. When installed into a similarly threaded bore, the interlock is perfected and clamping and radial drawing of the female threaded hole threads occurs, so that interlocking of the male screw threads and the female threaded bore threads are completed. This will have the effect of strengthening the threaded bore material as if there had not been a hole cut into it let alone the negative effect of having a standard, spreading fastener tightened into it. Reversal of the prior art spreading force will greatly increase the life of threaded holes which affects the life and serviceability of the part.
This invention utilizes a thread tooth profile that has the same side, root and crest surfaces parallel to each other but not necessarily to the length axis of the pin. It also uses a new concept of varying the pitch of either or both of the side flanks of the teeth. Changes to the pitch geometry of the threaded pin while maintaining a constant pitch within the threaded receiving hole by increasing either or both flank pitches can cause the threaded pin to be self-engaging as it is screwed into the receiving hole. This self-engagement will result in the threads of the pin being drawn deeper into the side wall of the threaded hole which when tightening occurs and, results in halting continued forward rotation (advancement) of the pin. The depth of installation can be controlled by slight changes to the width of the crest by changes to the pitch of upper and lower flanks of the thread teeth.
Different threaded products can be manufactured with predetermined effects based on desired outcome. Providing radial clamping force on certain threaded fasteners can have dramatic affects on the base material that they are threaded into. Spreading force causes cracks. Threaded holes that are too close to an edge are prone to cracking. Threaded holes in soft materials are prone to stripping. By using the present fastener, the forces engendered cause the material receiving the fastener to draw towards the fastener not away as in the prior art.
The instant invention is distinguished over the known prior art in a multiplicity of ways. As will be defined in the ensuing description of the preferred embodiment, the fastener is defined by a shaft or shank which may be cylindrical or tapered. A thread is spirally wound around the shaft. In section, the thread has a contour which diverges or bulges as it extends further outwardly from the shaft. Each section of the thread includes a root base, an upper flank and a lower flank which are girded on an outer exterior by a crest having a height. A root gap is interposed between adjacent teeth in line with the root base. The key to the instant invention involves the relationship of these various aspects of the fastener, one to the other and to its associated bore.
For example, assume the portion of the thread viewed in section appears as a dovetail. Each dovetail has an upper flank (nearer the fastener""s driving head or xe2x80x9ctopxe2x80x9d) and a lower flank. These threads are spirally formed about a shaft of the fastener. The upper flank preferably has a constant pitch whereas the lower flank has a pitch which varies along the length of the thread. By pitch it is meant thread density per unit length of shaft. In actuality, with a thread which is xe2x80x9cdovetailxe2x80x9d in section, the thickness of the thread will vary. More particularly, it is preferred that the pitch of the lower flank decreases as it goes from a xe2x80x9cbottomxe2x80x9d or second (lower) end of the fastener to the xe2x80x9ctopxe2x80x9d or first (upper) end of the fastener. By xe2x80x9cbottomxe2x80x9d it is meant an end of the shaft remote from a driving end. This means that the distance between adjacent, vertically aligned lower flanks of the thread increases as it moves from a bottom of the shaft to a top of the shaft. In this example, it is preferred that the upper flank""s pitch is held constant.
Two geometrical corollaries are obtained from the foregoing example. First, the root gap of the shaft that extends between facing upper and lower flanks will correspondingly increase in dimension from the top to bottom as successive root gaps are viewed from a top most portion (or first end of the threaded shaft) to a lower most or second end of the threaded shaft. Note that while the root gap increases, the root base decreases from top to bottom.
Second, the crest height which is defined by the outer wall of the crest that exists between each upper flank and lower flank on any given dovetail section will decrease in dimension and gets measureably smaller as it descends from the first top end of the shaft to the second bottom end.
By having these variations in the dovetail thread geometry, the dovetail threaded fastener delivers an increasing frictional gradient with respect to a dovetail threaded bore into which is to be received. Preferably, the bore which is to receive the threaded fastener has a constant pitch for both the upper flank and the lower flank. Preferably, the lower flank of the bore is held constant at the same pitch as the upper flank of the threaded fastener. The upper and lower flank angles of both the bore and fastener preferably are also set at respective, complemental constant angles. As a direct consequence, the dovetail threaded fastener meets a relatively minimal resistance when first being inserted into the threaded bore, but the resistance to rotation ramps up as the threaded fastener is advanced into the threaded bore due solely to varying the pitch of the threaded fastener""s lower flank. This results in substantially greater retention power.
The fastener according to the present invention enjoys particular utility in repairing cracks in cast material for example as evidenced by applicant""s previous U.S. Pat. Nos. 5,417,532, 5,499,892 and 5,379,505. It has been found by applicant that the ability to withstand separation along a crack line is at least three times greater with the dovetailed fastener than by using the structure delineated in applicant""s earlier patents. Nonetheless, the fastener exhibits similar performance enhancement in media other than cast metal, such as wood, metal, synthetics, etc.
In addition, the structure according to the present invention is appreciably stronger in retarding axial translation both of the fastener and the material within which the fastener is located. Further, the fastener has appreciably greater strength in resisting bending moments, resisting torsional flexure.
Accordingly, it is a primary object of the present invention to provide a new and novel fastener.
A further object of the present invention is to provide a device as characterized above which has a thread structure which when viewed in cross-section widens as it extends from a central shaft.
A further object of the present invention is to provide a device as characterized above which has greater resistance to axial forces, radial forces and bending moments.
A further object of the present invention is to provide a device as characterized as above which is to be received in a bore having a recess in which the geometry of the recess of the bore is substantially constant and does not vary as does the fastener.
A further object of the present invention is to provide a device as characterized above in which the variation in the thread geometry of the fastener allows initial easy starting of the threaded fastener into the associated bore and is subsequently followed by an increase in the rotating force required to incrementally advance the fastener.
A further object of the present invention is to provide a device as characterized above which is durable in construction, extremely safe to use and lends its self to mass production techniques.
Viewed from a first vantage point, it is an object of the present invention to provide a fastener, comprising in combination: a shaft having a first end including means to rotate the shaft, the shaft having a thread projecting from an outer periphery of the shaft and spiraling along its length, the thread defined by an upper flank, a lower flank, a root adjacent the shaft and a crest remote from the shaft, the thread defining a dovetail when viewed in section, the lower flank of the thread having a pitch which varies along the thread.
Viewed from a second vantage point, it is an object of the present invention to provide a fastener, comprising in combination: a shaft having a first end including means to rotate the shaft, the shaft having a thread projecting from an outer periphery of the shaft and spiraling along its length, the thread defined by an upper flank, a lower flank, a root adjacent the shaft and a crest remote from the shaft, the thread defining a dovetail when viewed in section, the crest having a crest height defined by an outer periphery subtended by one said upper flank and one said lower flank, where crest heights changes along the length of the thread.
Viewed from a third vantage point, it is an object of the present invention to provide a fastener, comprising in combination: a shaft having a first end including means to rotate said shaft, the shaft having a thread projecting from an outer periphery of the shaft and spiraling along its length, the thread dined by an upper flank, a lower flank, a root adjacent the shaft and a crest remote from the shaft, the thread defining a dovetail when viewed in section, the root having a root gap defined by facing upper and lower flanks, the root gap changes along the length of the thread.